In the optical communication system or optical measurement system, various optical devices are used including optical isolators, optical circulators, optical switches and optical attenuators. These optical devices are built with Faraday rotation devices so as to rotate the polarization plane. The Faraday rotation device is structured so as to apply an external magnetic field to the Faraday element (magnetic garnet single crystal having a Faraday effect), thereby controlling the Faraday rotation angle of a light ray which transmits through the Faraday element. In this case, there are a type in which a fixed magnetic field is applied to the Faraday element to keep the Faraday rotation angle constant (Faraday rotator) and a type in which a variable magnetic field is applied to the Faraday element to variably control the Faraday rotation angle (Faraday rotation angle varying device).
The conventional Faraday rotator is generally of a structure accommodating a Faraday element in a cylindrical permanent magnet. On the other hand, in the case of a Faraday rotation angle varying device, the structure generally has a Faraday element which is arranged between two ring-formed permanent magnets so as to apply a fixed magnetic field that is parallel with a light ray direction while a variable magnetic field is applied orthogonal to the light ray direction by an electromagnet, to change the resultant magnetic field of them and the magnetization direction of the Faraday element, thereby varying the Faraday rotation angle.
Meanwhile, the optical communication system or optical measurement system requires an optical attenuator to control the amount of transmission light. The optical attenuator is structured such that a polarizer and an analyzer are arranged on the optical axis in the front and rear (input and output sides) of a Faraday rotation angle varying device. The Faraday rotation angle varying device incorporated is to apply external magnetic fields in two or more directions to the Faraday element (magnetic garnet single crystal having a Faraday effect) and vary the resultant magnetic field thereof, thereby controlling the Faraday rotation angle of a light ray transmitting through the Faraday element. The variable optical attenuator variably controls the amount of light attenuation by controlling the Faraday rotation angle.
In the optical attenuator, the polarizer and the analyzer may use composite polarization prisms. However, the use of a composite polarization prism reduces the amount of incident light by the polarizer by half. Accordingly, it is usually rather practical to make a fiber-coupled type device in a polarization-independent type by using a wedge-shaped birefringent crystal (e.g. rutil single crystal).
The incident light, from an input fiber, turns into a collimated light in a lens and passes a polarizer, a Faraday element of a Faraday rotation angle varying device and an analyzer in this order, to be focused by a lens and coupled to an output fiber. The Faraday element is applied with a fixed magnetic field which is parallel with the light ray direction by a permanent magnet and with a variable magnetic field orthogonal to the light ray direction by an electromagnet. By changing the resultant magnetic field of these, and hence the magnetization direction of the Faraday element, the Faraday rotation angle is varied to thereby control the amount of the light passing through the analyzer.
Meanwhile, due to the wavelength division multiplex communication which has begun to be placed into practical application, the variable optical attenuator is required to decrease its wavelength dependence at around a particularly required amount of attenuation (particularly in a region having a great attenuation amount) depending upon a service state.
Concerning the reduction in wavelength dependence of a Faraday rotation angle, there is a proposal on a structure of a combination of a garnet single crystal basic film to vary the Faraday rotation angle and a garnet single crystal compensation film to render the Faraday rotation angle nearly constant. However, the problem of wavelength-dependent loss (attenuation amount change with a change of wavelength) has not yet been solved.
For example, in a wavelength division multiplex transmission system using erbium-added optical fiber amplifiers in multiple stages, a variable optical attenuator is used for a level adjustment of amplified signal light. At this time, the synthesized signal light causes a wavelength-dependent loss due to the optical attenuator, which worsens the gain flatness characteristic of the wavelength division multiplex system. For this reason, it is of an extreme important technical problem to suppress, to the minimum, the wavelength-dependent loss at the time of light attenuation.
As in the above, in accordance with the wavelength division multiplex communication that has begun to be placed into practical application, the optical devices in various kinds are required to have a small wavelength dependence. From the viewpoint of reducing the wavelength dependence of Faraday rotation angle, there is a structure of a combination of a garnet single crystal basic film and a garnet single crystal compensation film reverse in Faraday rotation direction to that and nearly constant in Faraday rotation angle.
Particularly, in the Faraday rotation angle varying device to be built in an optical attenuator, there is a need to decrease the wavelength-dependent loss besides the reduction in wavelength dependence. For this reason, there is a proposal of a structure in which a garnet single-crystal compensation film having a nearly constant Faraday rotation angle is arranged besides a garnet single-crystal basic film that Faraday rotation angle changes with varying the resultant magnetic field, wherein the basic film and the compensation film use those different in the sign of Faraday rotation angle, whereby the compensation film reduces the wavelength change amount of Faraday rotation angle through the basic film (JP-A-2000-249997).
In order to realize this, there is a necessity for a compensation film corresponding to a basic film. However, it is not necessarily easy to make a suitable compensation film corresponding to an arbitrary basic film. This is because, although the wavelength characteristic and temperature characteristic of the basic film and compensation film must be offset to a possible great extent, there is limitation in material design due to quite different compositions between the two.